1. Field of the Invention
The invention relates to an oxidation and sulfidation resistant chromium-niobium containing alloy, a mixture containing chromium and niobium and a chromium and niobium-containing article as well as to a method for improving the oxidation and sulfidation resistance of an article using a chromium and niobium-containing material.
2. Description of the Prior Art
It is well known that structural base metals such as iron (Fe), cobalt (Co) and nickel (Ni) are neither oxidation nor sulfidation resistant when exposed to oxidizing or sulfidizing environments at high temperatures. It is also well-established that the foregoing metals undergo sulfidation at rates orders of magnitude greater than the rates at which they undergo oxidation under comparable sulfur partial pressure and temperature conditions as reported by Kofstad, High Temperature Corrosion, Elsevier, London (1988), p. 429.
The pure element chromium (Cr) and chromium-containing alloys display high temperature oxidation resistance due in part to the formation of a Cr.sub.2 O.sub.3 scale, which allows only very slow diffusion of the reactant species involved in the oxidation process through the scale, even at high temperatures, thereby inhibiting the oxidative corrosion process. Niobium and its alloys exhibit poor oxidation resistance at high temperatures.
Pure, elemental niobium (Nb), known also as columbium (Cb), is known to display high temperature sulfidation resistance, analogous to the oxidation resistance of chromium and chromium-containing alloys, due in part to the formation of a passivating scale which can include a NbS.sub.2 scale as described in Strafford and Bird, "The Kinetics of Sulphidation of Niobium", J. Less-Common Metals, 68, (1979), 223. Attempts to capitalize on the sulfidation resistance of Nb by incorporating this element as an alloying addition to other base metals as described in Gleeson et al., "Effect of Nb on the High-Temperature Sulfidation Behavior of Cobalt", Oxidation of Metals, 31, (1989), 209-236; Chen et al., "High-Temperature Sulfidation Behavior of Ni-Nb Alloys", Oxidation of Metals, 31, (1989), 237-263 and Wang et al., "High-Temperature Sulfidation of Fe-Nb Alloys", Oxidation of Metals, 32, (1989), 273-294 have not resulted in alloys exhibiting acceptable sulfidation resistance. Additions of up to 40 wt. % Nb failed to form a continuous NbS.sub.2 protective layer in Ni, Co and Fe binary alloys. Further additions of molybdenum to form a ternary alloy improved sulfidation resistance only slightly.
Chromium and conventional chromium-containing alloys sulfidize catastrophically in the presence of sulfur. Even pre-oxidized chromium and chromium alloys having a Cr.sub.2 O.sub.3 scale are rapidly degraded by low sulfur levels present in H.sub.2 -H.sub.2 O-H.sub.2 S gas mixtures where oxygen partial pressure is of the order of 10.sup.-10 atm and sulfur partial pressure is of the order of 10.sup..sup.-6.5 atm as observed for a Fe-45 wt. % Cr alloy at 1173K.
Environments are frequently encountered, particularly in energy conversion system applications involving fuel-fired steam burners, coal-fired gas turbines, coal gasification and liquefaction and fuel gas desulfurization, as well as in the petrochemical industries, where both oxygen and sulfur are present at high temperatures. Materials used in such applications can be exposed to sharp variations in oxygen and sulfur concentration and can be exposed to gas mixtures containing both oxygen and sulfur as well as to gas mixtures which are either extremely oxidizing or extremely sulfidizing. Typical oxygen contents can be in the range of from about 1.times.10.sup.-10 atm to about 1.times.10.sup.-20 atm and typical sulfur contents can be in the range of from about 1.times.10.sup.-7 atm to about 1.times.10.sup.-10 atm. These harsh environments demand materials which can withstand extreme oxidizing conditions, extreme sulfidizing conditions as well as conditions which are simultaneously oxidizing and sulfidizing at high temperatures.
Thus, there exists a need for materials which exhibit oxidation resistance, sulfidation resistance as well as combined oxidation and sulfidation resistance at elevated temperatures and for a method for improving the oxidation resistance, sulfidation resistance and combined oxidation and sulfidation of articles at high temperatures.